I just replaced mine in my wm9 sub yesterday. I was in the same boat as you. This was my first attempt at replacing these items. I removed the old stem and measured it with calipers, it measured right at 20mm. I carefully held the stem with some very small needle nose pliers and unscrewed the crown from the stem. I placed the new stem next to the old stem and marked it 2mm longer than the old one,(easier to make the new one shorter, hard to make it longer!!!). Then I screwed the new stem into the crown and measured it with the calipers, in my case it was exactly 22mm. Yours may be different if it was screwed in more or less than mine. I then unscrewed the stem from the crown, and cut of 2mm with some small diagonal cutters. I applied a TINY bit of red loctite (not sure about this one) to the stem and screwed it into the stem and measured again. It measured 20mm. I reinstalled the stem into the watch/movement and it worked like a champ. I am not a watchmaker... I just figured if I make the old one the same length as the new one it should work and it did. I would like other members to chime in on my methods, so I"m not a complete HACK. I just wanted to share what worked for me....One watch nut to another. Hope this helps you.
Reelmin
Fish on!!!